1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device.
2. Description of the Related Art
In the process of manufacturing a semiconductor device, a metal impurity such as Fe (iron) or Ni (nickel) often mixes in a silicon substrate on which the semiconductor device is to be formed. If a metal impurity, especially a heavy metal impurity, is electrically activated in the silicon substrate, this considerably degrades the property of the semiconductor device.
Paragraphs 0017-0020 of Japanese Patent Laid-Open No. 7-7008 disclose a method for forming silicon oxide films 2 on the two faces of a substrate 1, removing the silicon oxide film 2 on the back face of the substrate 1 by etching, and forming silicon nitride films 3 and 60 on the front and back faces, respectively, of the substrate 1. The silicon nitride film 60 directly formed on the back face of the substrate 1 functions as a film for gettering in which a metal impurity is removed. Paragraphs 0021-0024 of Japanese Patent Laid-Open No. 7-7008 disclose a method for removing the silicon oxide film 2 formed on the back face of the substrate 1, and forming polysilicon films 50 and 55 on the front and back faces, respectively, of the substrate 1. The polysilicon film 55 directly formed on the back face of the substrate 1 functions as a film for gettering in which a metal impurity is removed. The above-mentioned two methods disclosed in Japanese Patent Laid-Open No. 7-7008 employ a technique for forming a gettering film on the back face of the substrate during a semiconductor device manufacturing step (by the so-called inline process) to getter a heavy metal impurity without adding a complicated step (paragraphs 0020 & 0024).
When isolation such as STI (Shallow Trench Isolation) is formed in accordance with the manufacturing method disclosed in Japanese Patent Laid-Open No. 7-7008, the polysilicon film formed on the back face of the substrate as a gettering film may immediately be removed. When the polysilicon film formed on the back face of the substrate as a gettering film is immediately removed, a sufficient gettering effect cannot be obtained.